A mixed cellular and D2D system allows devices to directly exchange information with each other, i.e. without passing through a base station and a core network as is common with conventional cellular communication systems. An exemplifying mixed cellular and D2D system, e.g. featuring D2D Proximity Services (D2D ProSe), was introduced with Release-12 (Rel-12) of Long Term Evolution (LTE) of the Third Generation Partnership Project (3GPP). With D2D ProSe, two main services for D2D communication are provided; a direct communication service and a direct discovery service.
With the direct communication service, the devices establish a direct user plane connection. The direct communication service primarily includes voice, such as Voice over Internet Protocol (VoIP), and file transfer, such as file transfer over File Transfer Protocol (FTP). With the direct discovery service, the devices transmit and monitor signals that enable them to be aware of proximity to another device or other devices.
Use cases for D2D ProSe include both Public Safety (PS) and commercial use.
In the PS applications, the direct communication service provides basic VoIP connectivity to other devices, independently of cellular coverage, such as LTE coverage, for the devices. Moreover, the direct discovery service may provide functions for detection of presence of peer devices, e.g. based on their professions or the like. Typically, in an emergency situation, it would be beneficial to easily find out if there is any doctor and/or nurse nearby.
In the commercial applications, the direct communication service provides simple push-to-talk functions, where communication typically is be one-to-many. Furthermore, the direct discovery service can broadcast advertisements, such as “pizza for 5$”, etc.
In a known mixed cellular and D2D system according to 3GPP Rel-12, a discovery message is sent from a device in order to enable other devices to find it and set up one or more D2D links. The discovery message is configurable with up to 3 HARQ retransmissions of the original discovery message. This means that the original discovery message can be repeated three times. Moreover, the HARQ retransmissions are autonomous, which means that the HARQ retransmissions are sent without reception of any HARQ feedback. The configuration of a number of HARQ retransmissions is cell specific, which means that all devices served by the same cell, or base station, will have the same number of HARQ retransmission attempts. On a per cell basis, it is thus possible to adjust the number of HARQ retransmissions. A disadvantage is though that it may be difficult to adjust the number of HARQ retransmissions for D2D links such that improved D2D communication may be achieved.